This invention relates to current generating electrochemical cells, and more particularly to current generating electrochemical cells wherein the anode material comprises an alloy of lithium and boron.
An electrochemical cell, is generally defined as a system consisting of an anode, a cathode, and an electrolyte which is in contact with, and separates the anode and cathode. In one of its simplest modes of operation an electrochemical cell undergoes an oxidation-reduction reaction to produce an electric current, with the electron transfer occurring through an external circuit. More specifically, oxidation takes place at the anode with the anode material losing electrons which are transmitted through the external circuit, while reduction takes place simultaneously at the cathode with the cathode material accummulating electrons from the external circuit.
Alkali metals, because of their low electronegativity, low equivalent weight and high conductivity are desirable for use as the anode material in electrochemical cells. Conversely, elements of high electronegativity and low equivalent weight such as the halogens and chalcogens, are most suitable as the cathode material in electrochemical cells. Electrochemical cells employing alkali metal anodes and halogen or chalcogen cathodes are characterized by high specific energy, as well as high specific power.
One of the most energetic electrochemical systems known is the lithium-chlorine fused salt system in which lithium, lithium chloride and chlorine gas, serve as the anode material, electrolyte and cathode material, respectively. In electrochemical cells using the lithium-chlorine fused salt system the lithium is normally constrained in a fiber or screen matrix of a lithium-wettable metal, such as iron, stainless steel or nickel, which matrix is mounted in a protective stainless steel shell. This arrangement is necessary because lithium metal is molten at the normal operating temperature of the cell. The chlorine gas is typically diffused into the cell through a porous carbon gas diffuser, which serves as a current collector.
The lithium-chlorine fused salt system has been the subject of very extensive investigation, and the electrochemistry of the system is now well known. Lithium-chlorine fused salt cells produce a high discharge voltage, the electrode reactions are simple and extremely rapid with little or no polarization, and the conductivity of the fused salt electrolyte is at least an order of magnitude greater than the conductivity of electrolytes in aqueous systems. A representative lithium-chlorine cell design is described in detail in U.S. Pat. No. 3,909,297 to Zeitner et al.
Most military applications of current generating electrochemical cells, such as in torpedoes, missiles and small underwater vehicles, require that the cell activation time must be very short, i.e., on the order of thirty seconds or less. During the course of research aimed at the development of a lithium-chlorine cell design having a self-contained heat source capable of activating the cell in thirty seconds or less, two problem areas were encountered involving the lithium anode. The first problem occurred when the cell was heated rapidly to its operating temperature. During the rapid heating the molten lithium flowed out of its matrix and formed a conductive bridge between the anode and cathode, thus shorting out the cell internally. The movement of the lithium out of the matrix is believed to be due in part to the large volume expansion of the lithium metal when the cell is heated rapidly from room temperature to about 600.degree. C.
The second problem involving the lithium anode resulted from attempts to design a rapidly activated lithium-chlorine cell having a self-contained means for hypergolic activation. Hypergolic activation involves placing reactive powdered mixtures, around the anode and on top of, or intermixed with, the electrolyte, which mixtures react exothermically with the chlorine gas to generate intense heat. The heat produced by this reaction raises the temperature of the cell and melts the electrolyte. Once the electrolyte is molten, the cell can be discharged.
Examples of reactive powdered mixtures suitable for initiating hypergolic activation in the manner described above are described in the aforementioned U.S. Pat. No. 3,909,297, as well as in U.S. Pat. application Ser. No. 385,926, filed on July 24, 1973 and now U.S. Pat. No. 4,026,725, in the name of R. A. Sutula, which is assigned to the assignee of the present invention. A particularly effective reactive powdered mixture for hypergolic activation is disclosed in a patent application entitled "Reactive Mixtures", Navy Case No. 57,376 Ser. No. 726,370 by R. A. Sutula, filed on even date herewith.
By providing the cell with means for hypergolic activation, cell activation times shorter than thirty to forty-five seconds are theoretically obtainable. In practice, the heat produced by the reaction of the aforementioned powdered mixtures with chlorine gas not only raises the temperature of the cell but also raises the temperature of the protective stainless steel shell containing the anode material to a point at which the stainless steel reacts directly with the chlorine gas. The additional heat produced from the stainless steel-chlorine reaction vaporizes some of the lithium, which in turn reacts directly with the chlorine gas, producing more heat. Eventually, the anode material is consumed by direct reaction with the chlorine gas and the cell fails to function.
The shortcomings of the prior art anode structures are apparent. The anode design is rather complex in that metal matrices and shells are necessary to constrain and protect the lithium metal. Such provisions are not completely effective, however, since, as mentioned above, when electrochemical cells containing such anode structures are subjected to rapid heating the anode does not perform consistently due to the flow of the molten lithium from the anode structure. Furthermore, the anode structures are not truly chemically compatible with the other materials used in the cell, particularly when the cell is subjected to the intense heat produced by hypergolic activation.
Consequently, a need exists for an anode material which can overcome the above described shortcomings of the prior art anodes which are presently being used in lithium-chlorine fused salt electrochemical cells.